[Illustration: Fig. 18. Examples of _Nostoc_. _N. Linckia_ Born. A, nat. size; B, small portion × 340; C, _N. coerulescens_ Lyngbye, nat. size (after West).] [Illustration: Fig. 19. Example of _Scytonema_ alga. _S. mirabile_ Thur. C, apex of a branch; D, organ of attachment at base of filament. × 440 (after West).] _Nostoc_ occurs in a few or all of the genera of Pyrenidiaceae, Collemaceae, Pannariaceae, Peltigeraceae and Stictaceae, and _N. sphaericum_ Vauch. (_N. lichenoides_ Kütz.) has been determined as the lichen gonidium. When the chains are broken up it has been wrongly classified as another alga, _Polycoccus punctiformis_. Fam. SCYTONEMACEAE. Trichomes of single-cell rows, differentiated into base and apex. Pseudo-branching arises at right angles to the main filament. SCYTONEMA Ag. Pseudo-branches piercing the sheath and passing out as twin filaments (Fig. 19); colour, golden-brown. This alga occurs in genera of Pyrenidiaceae, Ephebaceae, Pannariaceae, Heppiaceae, in _Petractis_ a genus of Gyalectaceae, and in _Dictyonema_ one of the Hymenolichens. Fam. STIGONEMACEAE. Trichomes of several-cell rows with base and apex; colour, golden-brown. STIGONEMA Ag. Stouter than _Scytonema_, with transverse and vertical division of the cells, and generally copious branching (Fig. 20). This alga occurs only in a few genera of Ephebaceae. _S. panniforme_ Kirchn. (_Sirosiphon pulvinatus_ Bréb.) has been determined as forming the gonidium. Fam. RIVULARIACEAE. Trichomes with a heterocyst at the base and tapering upwards, enclosed in mucilage (Fig. 21). [Illustration: Fig. 20. _Stigonema_ sp. × 200 (after Comère).] [Illustration: Fig. 21. Examples of _Rivularia_; A, B, C, R. _Biasolettiana_ Menegh.; D and E, _R. minutula_ Born. and Fl. A and D nat. size; B, C and E × 480 (after West).] RIVULARIA Thuret. In tufts fixed at the base and forming roundish gelatinous colonies; colour, blue-green. The gonidium of Lichinaceae has been identified as _R. nitida_ Ag. Algae belonging to one or other of these genera of Myxophyceae also combine with the hyphae of Archilichens to form cephalodia[279] and Krempelhuber[280] has recorded and figured a blue-green alga, probably _Gloeocapsa_, in _Baeomyces paeminosus_ from the South Sea Islands. They also form the gonidia in a few species and genera of such families as Stictaceae and Peltigeraceae. _b._ CHLOROPHYCEAE ASSOCIATED WITH ARCHILICHENS. The lichens of this group are by far the most numerous both in genera and species, though fewer algal families are represented. Fam. PROTOCOCCACEAE. Consisting of globular single cells, aggregated in loose colonies, dividing variously. [Illustration: Fig. 22. _Pleurococcus vulgaris_ Menegh. (_Protococcus viridis_ Ag.). _chl._ chloroplast; _p._ protoderma stage; _pa_, palmelloid stage; _py_, pyrenoid. × 520 (after West).]